The present invention relates to a microcircuit with two operating modes, i.e. a contact mode and a contactless mode.
At the present time, the market of chip cards and in a general way the market of electronic chips mounted on portable supports comprises two fields the one of applications called "contact applications" an the one of "contactless applications". In a near future, contactless chip cards should have an important development, when, simultaneously, a lot of contact cards will still be used. Thus, to rationalise the market of chip cards, it has been thought to develop microcircuits with both operating modes, contact or contactless, able to communicate with any kind of chip card reader.
U.S. Pat. No. 5,206,495 discloses a chip card microcircuit with two operating modes, contact or contactless, whose structure will be recalled. The reference numerals between square brackets refer to FIG. 3 of this document.
The microcircuit [2] of the prior art comprises contacts [3] for the operating mode with contact and induction coils [4, 5] for the contactless operating mode. The induction coils are associated with a rectifier circuit [2.1.1] to provide a DC supply voltage U1 in the contactless working mode. In the contact working mode, a DC supply voltage U2 is provided by a supply contact [I6].
Essentially, the prior art document proposes to send to a microcomputer [2.2], by means of a multiplexer [2.1.3], the signals received by the contacts [3] and the signals received by the coils [4, 5]. The signals received by the coils [4, 5] are applied to the multiplexer [2.1.3] by means of a converter [2.1.4]. The microcomputer [2.2] forms thus a means for processing the signal common to the two operating modes, and the multiplexer [2.1.3] forms a switching unit allowing the connection of the microcomputer to the contacts [3] or to the converter [2.1.4], depending on the operating mode of the microcircuit. The discrimination of the operating mode, for controlling the multiplexer [2.1.3], is performed by a comparator [2.1.2] with two inputs [E1, E2], providing at its output [E3] a discrimination signal applied to the multiplexer [2.1.3]. The comparator [2.1.2] receives on a first input [E2] a DC voltage U2 coming from the supply contact [I6] and on a second input [E1] a DC voltage U1 coming from the rectifier [2.1.1].
Although satisfactory at first sight, the prior art microcircuit presents various drawbacks in practice.
A first drawback lies in the detection of the operating mode by means of the above-mentioned comparator [2.1.2], which receives the supply voltages U1 and U2 on its inputs [E1, E2]. The voltages U1, U2 are also applied to the supply input [E4] of the comparator [2.1.2] by means of diodes [D1, D2] which are essential to avoid a short circuit between the two comparison inputs [E1, E2]. Now, these diodes cause a substantial decrease of the supply voltage received by the comparator, for example 1 Volt in a CMOS integrated circuit. Thus, in order that the comparator [2.1.2] is able to work, the minimal supply voltage U1 or U2 of the microcircuit must be 1 V above the minimal supply voltage of the comparator. As the voltage U1 provided by the rectifier [2.1.1] in a contactless mode depends on the amplitude of the induced voltage in the coils [4, 5], and therefore on the distance between the microcircuit and the magnetic field source (inductive coupling), the voltage decrease in the diode [D1] must be compensated by an increase of the inductive coupling and implies a substantial decrease of the maximal communication distance which can be permitted between a chip card and a card reader, with the same emitting power of the magnetic field. To have a better idea, in a contactless chip card supplied by induction at the standard frequency of 13.56 MHz and able to work with a very low voltage of about 1 V (CMOS technology), a voltage drop of 1 V in the diode [D1] represents a decrease of several centimetres of the maximal distance card/reader, the supply voltage U1 having to be at least equal to 2 V for being 1 V on the supply input [E4] of the comparator.
Another drawback of the prior art microcircuit is the manner to deal with an eventual ambiguity between the supply voltages U1, U2 of the contact mode and the contactless mode. By "ambiguity" is meant the case where, simultaneously, the supply voltages U1 and U2 are present. In particular, the applicant has discovered that the fingers of a user which touches the contacts during the contactless mode can inject into the microcircuit static electricity charges capable to disturb its working and even to block it. In the prior art document, it is provided that the output [E3] of the comparator, producing the signal which discriminates the operating mode, is preferably non ambiguous as regards as the value of the voltages U1, U2. However, the present invention is based on the following constatation: in order that the output of the comparator [2.1.2] is non ambiguous when voltages U1, U2 are present simultaneously, and at high levels, it is necessary to give arbitrarily priority to one of the two voltages only. If priority is given to the voltage U1 of the contactless mode to solve the ambiguity case linked to an interfering electrostatic voltage on the contacts, the output of the comparator must be at 1 when the voltage U1 is present, whatever the voltage U2 may be. If the output of the comparator depends only on the voltage U1, it can be concluded that it is not useful at all to provide a comparator. Finally, the idea of using a unambiguous comparator for determining the operating mode implies some questionings about its implementation.
Yet another drawback of the prior art microcircuit lies in the fact that, in order that the signals of the contactless mode which are present on the outputs [K1-K5] of the converter [2.1.4] are fully compatible with the signals coming from the contacts [I1-I6], as it is indicated in the prior art, it is necessary that the converter [2.1.4] transforms these signals into signals received according to the communication protocol of the contact mode. More precisely, as the signals received on the various contacts of a chip card comprise respectively, according to the standard ISO 7816, data in serial form, a clock signal, a reset signal, a supply voltage, it follows that the converter [2.1.4] must provide each of these signals in a standard serial form on its corresponding outputs [K1-K6]. Thus, practically, the converter [2.1.4] must be a "protocol converter", or "adapter", having a great complexity and a high manufacturing cost. Here also, the implementation of the idea of multiplexing the input signals towards a common means for processing the communications is subject to significant practical difficulties.